The array substrate is an importance component of the liquid crystal display device, and comprises a plurality of thin film transistors (TFTs). The active layer of the conventional thin film transistor is made of materials such as polysilicon, amorphous silicon etc. Compared with the active layer of silicon material, the metal oxide semiconductor (e.g., Indium gallium zinc oxide (IGZO) or Indium tin zinc oxide (ITZO)) has advantages of high mobility, good uniformity, transparency etc. Hence, the metal oxide semiconductor has been used more and more widely as the thin film transistor (metal oxide thin film transistor) of the active layer.
However, since the etching property of the metal oxide is similar as the metal serving as the source and the drain, the Indium tin oxide (ITO) serving as the pixel electrode, it might be damaged in the fabricating process of the source and the drain, as well as the pixel electrode. Therefore, it needs to form an etch stop layer (ESL) made of silicon nitride, silicon oxide, etc. on the active layer (at least on a conductive area in the active layer when the thin film transistor is turned on), so as to prevent the active layer from being damaged in the subsequent process. An additional patterning step is required for forming the etch stop layer, this results in increase of fabricating steps of the array substrate using the metal oxide thin film transistor, and the processes become complex (generally 6 to 7 masks are required).